This invention relates to the removal of a chromium oxide coating from a surface of an article and, more particularly, to the removal of the chromium oxide coating from a nickel-base superalloy article that has been operated in a gas turbine engine.
In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine and compressed by an axial-flow compressor. The axial-flow compressor includes a number of compressor stages. Each compressor stage has a plurality of compressor blades mounted to a compressor disk, which in turn is mounted to a rotating shaft.
In many early versions of gas turbine engines, the compressor blades were made of an uncoated metal. As the technology of gas turbine engines has advanced and the temperatures of operation have increased, it has become necessary to coat the compressor blades to inhibit oxidation of the metal during extended service. The coatings need not be as protective and as resistant to the effects of the combustion gas at high temperatures as the environmental coatings and thermal barrier coatings used on the turbine blades, but they must provide oxidation protection at intermediate temperatures. Chromium oxide coatings cured in the presence of an inorganic binder have been selected for use to coat the compressor blades of the high-pressure compressor stages of some engines.
After the gas turbine engine has been operated in service for a period of time, it is typically disassembled for routine inspection. Inspections may also occur if problems are observed. In the inspection, the compressor blades are removed from the compressor disk. Some areas of the surfaces of the compressor blades are usually covered with contaminant materials such as dirt, oxide scale, and hydrocarbon residue, as well as the chromium oxide coating. To inspect the compressor blades properly, it is necessary to remove the contaminant material covering their surfaces. In one approach, the contaminant material may be removed with a mechanical technique such as grit blasting or glass bead blasting, but these mechanical techniques may compromise the subsequent inspection or harm the base metal of the compressor blade.
At the present time there is no chemical method for removing the surface material found on nickel-base superalloy compressor blades that are initially covered with the chromium oxide coating and then operated in service, and there is need for such a technique. The present invention fulfills this need, and further provides related advantages.
The present invention provides a technique for removing a chromium oxide coating from an article. Overlying contaminant materials resulting from exposure to a gas turbine environment, such as dirt, oxide scale, and hydrocarbon residue, are also removed. The present approach is based primarily on chemical cleaning, without the use of grit or bead blasting. It is therefore relatively gentle on the underlying base metal of the article being cleaned, and reaches areas not accessible by line-of-sight techniques. It is suitable for mass production processing.
A method of removing a coating from a surface, such as the surface of a compressor blade, comprises the steps of providing an article having the coating comprising chromium oxide thereon, cleaning the article in an alkaline degreasing/rust removal solution at a degreasing/rust removal temperature, preferably from about 180xc2x0 F. to about 200xc2x0 F., scale conditioning the article in an alkaline permanganate conditioning solution at a scale-conditioning temperature, preferably from about 160xc2x0 F. to about 200xc2x0 F., and contacting the article to an acidic stripping solution comprising hydrochloric acid and an etching inhibitor at a stripping temperature, preferably from about 130xc2x0 F. to about 140xc2x0 F. Preferably, the article is washed in water after each of these steps, and optionally gently brushed to remove loose material from the surface.
The method may include an additional step, after the step of contacting, of rinsing the article and brushing the article to remove any loose material thereon. If the described procedure is not sufficient to remove all of the chromium oxide coating, it may include additional steps, after the step of rinsing, of second contacting the article to the acidic stripping solution at the stripping temperature, and second rinsing the article and second brushing the article.
The present approach may be applied to a wide variety of articles. Desirably, the article is a superalloy article having the coating thereon, and most preferably it is a superalloy article such as a compressor blade that has been operated in a gas turbine engine.
The alkaline degreasing/rust removal solution preferably comprises sodium hydroxide with a pH of greater than about 14. The alkaline permanganate conditioning solution preferably comprises permanganate in a concentration of at least about 25 percent by weight and has a pH of greater than about 14. The acidic stripping solution preferably comprises hydrochloric acid in a concentration of from about 85 to about 135 grams per liter. The inhibitor preferably comprises acetic acid, isopropyl alcohol, hexamethylenetetramine, and propargyl alcohol in a concentration of about 0.3 percent by volume of the acidic stripping solution.
The present approach removes the contaminant material, if any, and the chromium oxide from the surface of the article. It does not mechanically alter the surface of the article, so that subsequent inspections of the article are not impeded. The only mechanical processing of the surface is an optional gentle brushing.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.